The invention relates to a method and a device for determining the internal temperature of an electrochemical energy storage device, particularly for a motor vehicle.
The energy of an electrochemical energy storage device depends on its operating temperature. This is particularly true for—but not only for—energy storage devices which use lithium ion storage cells.
An energy storage device used in the motor vehicle field typically has a plurality of storage cells which are electrically connected to each other in series and/or in parallel in order to be able to provide a prespecified output voltage and a prespecified output current. In the storage modules developed to date, the storage cells are based on the lithium ion technology mentioned above. These are ideally operated in a defined temperature range. This can be defined, by way of example, between the temperatures of +5° C. and +40° C. If the operating temperature of the storage cells exceeds the upper temperature threshold, accelerated aging results, such that it is frequently not possible to comply with a required operating life. If, in contrast, the storage cells are operated below the lower temperature threshold, the capacity of the cells is sharply reduced. In addition, it is not possible to operate the storage cells efficiently in this temperature range. When energy storage devices are used in the field of motor vehicles, the temperature thereof is regulated for this reason.
In order to make it possible to regulate the temperature of storage cells precisely and efficiently, it is necessary to have the most precise possible knowledge of the actual temperature—meaning the internal temperature of the storage cells. It is then possible to carry out the cooling or heating of the storage cells on the basis of the determined current temperature of the storage cells. The detection of the temperature of the energy storage device is most commonly realized via a temperature sensor on the surface of the housing of the energy storage device or on a single storage cell. The temperature value measured at this location does not, however, correspond to the actual internal temperature of the electrochemical energy storage device. For this, a direct measurement of the internal temperature of a cell chamber would be necessary. However, a measurement of the internal temperature of a storage cell involves a great deal of constructive complexity. First, the process for manufacturing the energy storage device would be more involved, for example due to the routing of cable for the temperature sensor. Secondly, additional measures would need to be taken in order to meet the demands for tight sealing of the affected storage cell and/or of the energy storage device as a whole.
The problem addressed by the present invention is that of providing a method and a device by which it is possible to determine the internal temperature of an electrochemical energy storage device in a simpler manner.
These problems are addressed by providing a method, according to the invention, for the determination of the internal temperature of an electrochemical energy storage device, particularly for a motor vehicle, wherein the internal temperature of a cell winding of the electrochemical energy storage device is determined by a computer in a control device of the energy storage device. The computer utilizes a thermal model for the electrochemical energy storage device, said model being saved in the control device.
The invention also creates an electrochemical energy storage device, particularly for a motor vehicle, having a control device, wherein a thermal model of the electrochemical energy storage device is saved in the control device, and wherein the energy storage device is designed to execute the method according to the invention.
By way of the invention, it is possible to dispense with a temperature sensor used for the direct measurement of the internal temperature of the electrochemical energy storage device, which results in a benefit for the constructive embodiment of the energy storage device. By simulating the internal temperature using a thermal model for the energy storage device, it is possible, on the one hand, to realize the most precise possible temperature regulation of the storage cells of the energy storage device. A more homogeneous and warmer operation of the energy storage device over time, as a result of a precise temperature regulation, leads to a more efficient operation of the energy storage device overall, without posing the risk of damage to the storage cells of the energy storage device. Likewise, the risk of such temperature thresholds of storage cells of the energy storage device being exceeded is minimized, wherein the same leads to a more rapid aging. A further advantage of simulating the internal temperature of the energy storage device is that a more precise temperature signal can be used as the input signal for additional aspects of a status recognition function, such as a charge state recognition, by way of example. As a result, the precision of further status estimations is also increased.
The thermal model of the energy storage device is advantageously established on the basis of the thermal capacities and the thermal resistances. In this case, a thermal resistor is disposed between two of the thermal capacitances. The thermal capacitances represent components of the energy storage device, such as the cell winding of one or multiple storage cells, the housing of a storage cell, a connector terminal of the storage cell, a cooling device, etc., by way of example. By use of the thermal resistance between two of the thermal capacitances, a measure for the heat conductance from one thermal capacitance to the other thermal capacitance is taken into account.
In particular, the thermal model takes into account the thermal capacitances for the cell winding of at least one storage cell of the energy storage device, for the housing of the storage cell(s), for at least one connector terminal of the storage cell(s), and optionally for the cooling device. The incorporation of the cooling device as a thermal capacitance in the thermal model is optional because the cooling device can be switched on or off according to the regulation. If the cooling device is not active, then it can be left out of the thermal model. Only if the cooling device is in operation is it necessary to take into account the thermal capacity thereof.
It is also advantageous if a thermal dissipation loss of the energy storage device is worked into the thermal model. The thermal dissipation loss is determined by measurement from a detected current in the energy storage device, by way of example. The incorporation of the thermal dissipation loss of the energy storage device is significant if the temperature of the cell winding of one of the storage cells is determined as part of the simulation of temperature, because this is influenced by the thermal dissipation loss.
It is also advantageous if at least the temperature of one component of the energy storage device which represents one of the thermal capacitances excluding the cell winding is determined by measurement and is incorporated into the thermal model. A temperature which is simple to measure is preferably included—for example the temperature on the outside of the housing. As an alternative, the temperature at a connector terminal can be detected by means of measurement—as can the temperature of the cooling device.
For the thermal model, heat volumes exchanged between two adjacent thermal capacitances are determined iteratively for a prespecified time interval from the temperature difference between the two thermal capacitances, using the following formula:
                              Q                      i            →                          i              +              1                                      =                                                            T                i                            -                              T                                  i                  +                  1                                                                    R                              th                ,                                  i                  →                                      i                    +                    1                                                                                ⁢          Δ          ⁢                                          ⁢          t                                    (        1        )            
Next, for the thermal model, the temperatures for the thermal capacitances for each prespecified time interval are determined iteratively from the determined heat volume, using the following formula:
                                          T            i                    ⁡                      (                                          t                0                            +                              Δ                ⁢                                                                  ⁢                t                                      )                          =                                            T              i                        ⁡                          (                              t                0                            )                                +                                    1                              C                                  th                  ,                  i                                                      ⁢                                          ∑                                  k                  =                  1                                n                            ⁢                              Q                k                                                                        (        2        )            
The method according to the invention is therefore based on the iterative determination of heat flows for each thermal capacitance, wherein the corresponding temperatures can then be calculated therefrom. Because the method according to the invention is determined “online”—meaning in real-time during the operation of the control device of the energy storage device, it is necessary to make an initial determination of the starting values of at least some of the temperatures, said starting values being used in the simulation, when the control device is re-started. In this process, a differentiation must be made as to whether the restart of the control device takes place within a prespecified threshold for the downtime phase, or if the restart takes place after the prespecified threshold for the downtime phase has been exceeded. In the latter case, it can be assumed that all of the components of the energy storage device have the same temperature value. As such, the at least one temperature value of the control device, the same determined by making a measurement after the start of the control device, can also be used for the temperature values of the other components. As an alternative, the initial condition can be calculated as a continuous function of the downtime phase—for example in the form of a decay curve.
If the prespecified value for the downtime phase has not yet been reached upon the restart of the control device, and therefore the restart of the energy storage device, then the components of the energy storage device have different temperature values. In this case, it is advantageous if, after a period of downtime of the energy storage device, and up to a prespecified maximum time, starting temperature values for the temperatures of the components, said temperatures representing the thermal capacitances and also being impossible to detect by measurement, or being not detected by measurement, are estimated by a computer for use in the thermal model. In this way, it is possible to minimize a starting-value error in the determined temperature following every downtime phase.
The minimization of the offset error in this case can be estimated mathematically prior to the simulation carried out in real-time. In a first variant, the following steps can be carried out for the estimation of a starting temperature value following the downtime phase. First, the relevant temperature values last determined by a computer before the downtime phase are saved, along with one measured temperature value at another component of the storage cells. At the end of the downtime phase, a temperature difference between the saved, measured temperature value and the temperature value determined by a computer is calculated, wherein the temperature value determined by a computer is corrected before the subtraction, by means of a prespecified decay curve and the duration of the downtime phase. The temperature difference is added to the temperature value measured at the end of the downtime phase, wherein the resulting sum gives the current temperature value.
In addition, for the fastest possible correction of the residual starting-value error in the starting phase of each simulation, and also for the purpose of minimizing the difference between the simulation and reality, a regulatory observer can be implemented during the entire period of the simulation. By means of the regulatory observer, a measured surface temperature can advantageously be incorporated in the thermal model at the same time. By way of example, this can be carried out for the current-tap terminal at a given cooling device temperature.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.